1. Field of the Invention
The present invention relates to a Passive Optical Network (PON), and more particularly to a Wavelength Division Multiplexing (WDM) PON.
2. Description of the Related Art
An optical fiber cable is effective means for processing a large amount of information at ultra high speed network, such as a Fiber To The Curb (FTTC), a Fiber To The Home (FTTH), etc. A widely used PON includes one central office for providing a service to a plurality of Optical Network Units (ONUs). A PON may employ a WDM scheme, a Time Division Multiplexing (TDM) scheme and a Subcarrier Multiplexing (SCM) scheme. In the WDM scheme, a plurality of lights with different wavelengths are assigned to each ONU. In the TDM scheme, a light with the same wavelength is split according to the time slots and the split time slots are assigned to each ONU. In the SCM scheme, a light with the same wavelength is split according to different frequencies, and the split frequencies are assigned to each ONU.
FIG. 1 illustrates a SCM/WDM PON according to the prior art. As shown, the PON 100 includes a central office 110, a remote node 120, and a plurality of ONUs 130-1 to 130-M.
The central office 110 includes a plurality of downstream light sources 111-1 to 111-N for generating downstream optical signals with different wavelengths; a plurality of upstream receivers 112-1 to 112-N for detecting data from an upstream optical signal with a corresponding wavelength; a multiplexer/demultiplexer 114 for multiplexing the downstream optical signals to output the multiplexed downstream optical signals to the remote node 120, and demultiplexing multiplexed upstream optical signals; and wavelength-selective couplers 113-1 to 113-N.
Each of the downstream light sources 111-1 to 111-N generates a downstream optical signal including SCM downstream channels. The upstream receivers 112-1 to 112-N detect necessary data from SCM upstream channels constituting an upstream optical signal with a corresponding wavelength. The wavelength-selective couplers 113-1 to 113-N couple the corresponding downstream light sources 111-1 to 111-N and the corresponding upstream receivers 112-1 to 112-N to the multiplexer/demultiplexer 114. The multiplexer/demultiplexer 114 multiplexes downstream optical signals to output the multiplexed downstream optical signals to the remote node 120, and demultiplexes upstream optical signals multiplexed by the remote node 120 to output the demultiplexed upstream optical signals to each of the upstream receivers 112-1 to 112-N through the corresponding wavelength-selective couplers 113-1 to 113-N.
The remote node 120 includes a multiplexer/demultiplexer 121 for demultiplexing multiplexed downstream optical signals and multiplexing the upstream optical signals, and a plurality of optical splitters 122-1 to 122-N for splitting the demultiplexed downstream optical signal with a corresponding wavelength according to intensities, and outputting the split optical signals to the corresponding ONUs 130-1 to 130-M.
Each of the ONUs 130-1 to 130-M includes a wavelength-selective coupler 131 linked to each corresponding optical splitter 122-1 to 122-N of the remote node 120, a downstream receiver 132 for receiving SCM channels belonging to an assigned frequency band from received downstream optical signals, and an upstream light source 133 for generating upstream optical signals. The downstream receiver 132 further includes a filter for detecting a channel of the assigned frequency band. Further, the upstream light source 133 loads data on an upstream channel with an assigned frequency to transmit the loaded data, and the optical splitter of the remote node combines upstream channels input from corresponding ONUs into an upstream optical signal.
However, a conventional ONU currently must use an expensive analog distributed feedback laser. That is, an ONU must pay increased installation cost.